Biteau, J.

Deligny, O.

Ghia, P.

Huege, Tim

Lhenry-Yvon, I.

English

Pierre Auger Collaboration

KITopen

Radio detection of cosmic rays with the Auger Engineering Radio ArrayTim Huege1,3,∗ on behalf of the Pierre Auger Collaboration2,∗∗1Karlsruhe Institute of Technology (KIT), Institute for Nuclear Physics, Postfach 3640, 76021 Karlsruhe, Germany2Observatorio Pierre Auger, Av. San Martín Norte 304, 5613 Malargüe, Argentina3also at: Astrophysical Institute, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, BelgiumAbstract. The Auger Engineering Radio Array (AERA) complements the Pierre Auger Observatory with 150radio-antenna stations measuring in the frequency range from 30 to 80 MHz. With an instrumented area of17 km2, the array constitutes the largest cosmic-ray radio detector built to date, allowing us to do multi-hybridmeasurements of cosmic rays in the energy range of 1017 eV up to several 1018 eV. We give an overview ofAERA results and discuss the significance of radio detection for the validation of the energy scale of cosmic-ray detectors as well as for mass-composition measurements.1 IntroductionThe Pierre Auger Observatory [1] exploits hybrid detec-tion of extensive air showers measured at the same timewith multiple detection techniques for optimum determi-nation of the energy and mass of cosmic ray primaries.Following this spirit, its baseline surface detector (SD)and fluorescence detectors (FD) have been complementedby an array of radio detectors. This Auger EngineeringRadio Array (AERA) was deployed in the region wheremany enhancements, focused in particular at the detectionof air showers with energies in the EeV energy regime,have been set up.In this article, we briefly review the characteristics andcapabilities of the radio detectors of the Pierre Auger Ob-servatory and discuss how they contribute valuable infor-mation in particular for the determination of the energyscale of cosmic rays as well as mass composition studies.2 The Auger Engineering Radio ArrayWhen work on AERA began in 2008, the nature and char-acteristics of the radio emission from extensive air show-ers were still largely unclear — unlike today, where theemission physics and signal characteristics are very-wellunderstood [2, 3]. Furthermore, many questions regardingthe best strategy for the measurement of these pulsed radiosignals were still open. The goal of AERA, illustrated bythe explicit mentioning of “engineering” in its name, wasto explore different options for implementing a radio de-tector that can be scaled to areas significantly larger than akm2.These efforts resulted in the setup of a 17 km2 arrayof radio detectors. The individual detector stations are au-tonomous and can thus be freely spaced. Many of the ex-∗e-mail: tim.huege@kit.edu∗∗Full author list at http://www.auger.org/archive/authors_2018_10.htmlperimental challenges, for example related to power sup-ply, communications and data acquisition, were indeed re-lated to this autonomous design, which is a key and uniquefeature of AERA that differentiates it from purely cabledsetups for radio detection of cosmic rays.The layout of the array is depicted in Figure 1. The ra-dio antennas are co-located with the 750 m surface detec-tor array, the Coihueco and HEAT fluorescence telescopes,and the AMIGA underground muon detectors. This com-bination of detectors at the same location maximizes theinformation gathered on each individual air shower bymulti-hybrid detection.The antennas have been set up on a graded array, withgrid constants of 144, 250, 375 and 750 meters, prob-ing different energy and zenith angle ranges. Two differ-ent types of antennas, Logarithmic Periodic Dipole An-tennas (LPDAs) [4, 5] and “butterfly antennas” [4] are inuse. Both measure in the frequency band from 30 to 80MHz. Furthermore, two different types of digital elec-tronics, both of them capable of “internal” triggering onradio signals and one of them, in addition, allowing databuffering for up to 8 seconds and thus exploitation of ex-ternal triggering by the SD and FD, have been developedand deployed. A reference “beacon” transmitter is used toachieve time synchronization of the autonomous detectorswithin two nanoseconds [6].AERA started taking data in 2011 and has been final-ized in its current form in 2015. Its current dataset encom-passes more than 10,000 extensive air showers measuredwith both the SD and the radio antennas. In the following,we give a concise overview of the most important resultsderived with AERA up to now.3 Determination of the cosmic-ray energyOne of the key parameters to characterize cosmic-ray pri-maries is their energy. With AERA, we have demonstrated© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 210, 05011 (2019) https://doi.org/10.1051/epjconf/201921005011UHECR 20181 kmWater Cherenkov detectors (SD)AERA LPDAAERA butterfly, ext+int trigAERA butterfly, int trigAMIGA Unitary CellbeaconFD field of viewHEAT field of viewFigure 1. Map of the Auger Engineering Radio Array. 150 radio-detector stations cover a total area of 17 km2. Further explanationsare given in the text.that this energy can be determined with a resolution of17% or better [7]. Many different approaches exist to de-termine the cosmic-ray energy from radio measurements[2]. In AERA, we determine the energy fluence, i.e. theenergy deposited in the form of radio signals per unit area,at the locations of our radio detector stations. Exploitingour knowledge of the radio-emission pattern, we can theninterpolate and finally integrate the energy fluence to de-termine the total energy deposited on the ground in theform of radio signals, the radiation energy.After correction for the orientation with regard to thegeomagnetic field, this radiation energy is closely corre-lated with the energy in the electromagnetic cascade ofthe air shower, following a quadratic scaling because ofthe coherent nature of the radio emission. This is shownin Figure 2. As the radio emission undergoes no signifi-cant absorption or scattering in the atmosphere and con-stitutes a calorimetric quantity, it is very well suited forcross-calibration of cosmic-ray detectors via radio mea-surements [8]. Comparing the measured radiation energyto the one predicted by first-principle Monte Carlo simula-tions [9] also provides the opportunity for an independentcross check and validation of the energy scale of the PierreAuger Observatory [10], which is based on measurementsof the fluorescence detectors.4 Mass composition sensitivityThe depth of maximum Xmax of an extensive air shower isa mass-sensitive parameter that is directly accessible withfluorescence measurements. Radio signals from air show-ers also have sensitivity to Xmax due to the forward-beamednature of the emission as well as its enhancement on aCherenkov ring introduced by the non-unity refractive in-dex of the atmosphere [2]. Deeper showers thus produce a1017 1018 1019ECR [eV]105106107108109EAuger30−80MHz/sin2α[eV]A · 107eV(ECR/1018eV)B, A = 1.58± 0.07, B = 1.98± 0.043 - 4 stations with signal≥ 5 stations with signalFigure 2. After correction for the angle of incidence with respectto the geomagnetic field α, the energy deposited on the groundin the form of radio signals, the radiation energy, scales quadrat-ically with the energy of the primary cosmic ray. With AERA,an energy resolution of 17% has so far been demonstrated [8] forevents detected in at least 5 radio-detector stations with signal.steeper signal falloff with lateral distance from the impactpoint than shallower showers. In addition, pulse shape andpolarization information can be exploited.We have demonstrated that by comparison of the ra-dio signals of measured air showers with per-event MonteCarlo simulations, Xmax can be determined and is in agree-2EPJ Web of Conferences 210, 05011 (2019) https://doi.org/10.1051/epjconf/201921005011UHECR 2018400 500 600 700 800 900 100011001200Xmax FD in g/cm2400500600700800900100011001200Xmax RD in g/cm2analysis Aσ= 43.5 ± 4.1, µ = 2.8 ± 4.1400 500 600 700 800 900 100011001200Xmax FD in g/cm2400500600700800900100011001200Xmax RD in g/cm2analysis Bσ= 49.6 ± 5.7, µ = 12.9 ± 5.7200 100 0 100 200Xmax RD - Xmax FD  in g/cm20123456#200 100 0 100 200Xmax RD - Xmax FD in g/cm20123456#Figure 3. The depth of shower maximum Xmax as determinedwith AERA correlates well with the Xmax value determined fromthe Auger fluorescence detectors [11]. The combined FD-radioresolution is approximately 45 g/cm2, and the radio-only resolu-tion is of order 35-40 g/cm2.ment with the values measured for the same showers withthe fluorescence detectors [11], see Figure 3. The com-bined FD-radio resolution is approximately 45 g/cm2, i.e.the radio-only resolution is of order 35-40 g/cm2. This islikely limited by the sparsity of the radio array, as mea-surements with much denser radio arrays have yieldedXmax resolutions below 20 g/cm2 [12].Another option for mass composition studies with ra-dio antennas lies in their pure measurement of the en-ergy in the electromagnetic component of an air shower.Combined with a measurement of the muon content of airshowers, for example by the AMIGA underground muondetectors, this allows mass-composition studies through acompletely different method. This approach has particu-lar potential for inclined air showers, for which a classicalmeasurement of the electron number at the ground is nolonger feasible (the electromagnetic cascade dies out be-fore reaching the ground due to the large atmospheric massoverburden). As radio signals are not significantly attenu-ated in the atmosphere, radio detection at these geometriesallows the determination of the energy in the electromag-netic cascade, while particle detectors such as those of theSD will provide a clean measurement of the muonic cas-cade. The sensitivity for mass-composition measurementsarising from this complementarity is illustrated in Figure4.5 Radio measurements of inclined airshowersThe radio emission from extensive air showers is stronglyforward-beamed. For showers with near-vertical inci-dence, for which the shower maximum is located at a fewkilometers above sea level, the emission thus illuminates a0 10 20 30 40 50 60 70 80zenith angle in deg0.0.51.01.52.0.5figureofmeritproton-ironseparationpowerFOM = |µP−µFe|√σ2P+σ2Feρ600µ /√SρθRD ρ600µ / Ne XmaxFigure 4. A simulation study of the separation power betweenproton- and iron-induced air showers shows that combined mea-surements of the radiation energy, S ρθRD, and the muon content at alateral distance of 600 meters, ρ600µ , provide very good mass com-position sensitivity at all zenith angles [13]. In contrast, com-bined measurements of the electron number Ne and the muoncontent lose separation power towards higher zenith angles, asthe electromagnetic cascade dies out before the shower reachesthe ground. Detector effects are not included in this study.rather small area on the ground, of order hundreds of me-ters in diameter [14]. The area does not grow significantlywith the energy of the primary particle because of the verysteep lateral falloff of the radio signal and the even deeperdepth of maximum for higher particle energies. As a con-sequence, radio detector arrays aiming to measure near-vertical air showers need a dense antenna spacing with agrid constant of at most a few hundred meters. Scalingsuch arrays significantly beyond the size of AERA is thusnot cost-effective. For near-vertical showers, the energyreach is therefore limited to below 1019 eV, simply becausethe instrumentable area is limited.In contrast, for inclined air showers, due to the vastlyincreased atmospheric mass overburden, the shower max-imum is typically dozens of kilometers away from aground-based antenna array. Consequently the emission isbound to be distributed over a much larger area [14]. Thishas recently been confirmed experimentally with AERAdata [15] as presented in Figure 5: The distance of theradio detector station farthest from the air-shower axiswhich still measured a clear radio pulse increases signif-icantly with the air-shower zenith angle. At the high-est zenith angles, the lateral extent of the radio-emissionfootprint can exceed two kilometers in the shower plane.Projection to the ground then yields areas of more than2 km× 2 km× sec(83◦) × π = 103 km2 illuminated withmeasurable radio signals. This allows radio detection ofinclined air showers with very sparse radio arrays. Infact, we have demonstrated that a grid constant of 1.5 km,the same as that of the Auger SD, is sufficient for thedetection of inclined air showers [15]. For inclined airshowers, radio detection then provides a pure measure-ment of the electromagnetic component of the air shower,3EPJ Web of Conferences 210, 05011 (2019) https://doi.org/10.1051/epjconf/201921005011UHECR 2018° / SDθ60 65 70 75 80 85Axis distance of furthest station / m05001000150020002500Figure 5. With increasing zenith angle, the farthest distance from the shower axis at which a detectable radio signal was measured byAERA grows significantly. At the highest zenith angles, detectable radio signals illuminate areas larger than 100 km2.while the SD measurements provide the pure muon con-tent. As described above, this opens the possibility formass-composition studies of inclined air showers.Motivated by these findings, we will equip each water-Cherenkov detector of the SD with one radio antenna aspart of the ongoing AugerPrime upgrade [16]. The detailsof this endeavor are described in a dedicated article [17].6 ConclusionsThe Auger Engineering Radio Array, an array of 150 au-tonomous radio detector stations covering a total area of17 km2, measures extensive air showers in the energyrange of 1017 eV to several 1018 eV via their radio emis-sion in the 30 to 80 MHz band. AERA explored many dif-ferent approaches for the experimental design as well asseveral analysis strategies, and has made important contri-butions to leading the radio detection technique from pio-neering prototypes to maturity.In particular, we have demonstrated that the energy de-posited in the form of radio waves on the ground, the ra-diation energy, can be measured accurately and is a reli-able estimator for the energy in the electromagnetic cas-cade of an extensive air shower. This quantity has a well-defined physical meaning and can be measured and com-pared among radio detectors worldwide. As such, it is avery useful means to cross-calibrate the energy scale ofdifferent cosmic-ray experiments, and can even be used tovalidate the energy scale on the basis of first principle cal-culations.Mass composition sensitivity is also present in radiosignals. We have successfully reconstructed the depth ofshower maximum of air showers from radio measurementswith AERA, and they are in agreement with measurementsfrom the Auger fluorescence detector. Another way to de-termine the mass of primary cosmic rays with the help ofradio measurements lies in combining radio detection withmuon measurements, as are provided for near-vertical ge-ometries by the Auger AMIGA detectors and for inclinedair showers by the Auger surface detector.Finally, we have established experimentally that in-clined air showers illuminate areas of dozens or even morethan a hundred km2 with measurable radio signals. Hence,arrays with grid constants of a kilometer or larger can mea-sure inclined air showers with reasonable antenna multi-plicity. This allows radio measurements up to ultra-highenergies, and yields very complementary information tothat provided by the water-Cherenkov detectors of the SD.Consequently, as part of the AugerPrime upgrade, we willdeploy a radio antenna on top of each water-Cherenkov de-tector to extend the composition-sensitive measurementsof AugerPrime to inclined air showers. At the same time,we will continue to operate AERA, which is focused atenergies around the EeV scale.References[1] A. Aab et al. (Pierre Auger Collaboration), Nucl. In-strum. Meth. A 798, 172 (2015)[2] T. Huege, Physics Reports 620, 1 (2016)[3] F. G. Schröder, Progress in Particle and NuclearPhysics 93, 1 (2017)[4] P. Abreu et al. (Pierre Auger Collaboration), JINST7, P10011 (2012)[5] A. Aab et al. (Pierre Auger Collaboration), JINST12, T10005 (2017)[6] A. Aab et al. (Pierre Auger Collaboration), JINST11, P01018 (2016)[7] A. Aab et al. (Pierre Auger Collaboration), Phys.Rev. D 93, 122005 (2016)[8] A. Aab et al. (Pierre Auger Collaboration), Phys.Rev. Lett. 116, 241101 (2016)4EPJ Web of Conferences 210, 05011 (2019) https://doi.org/10.1051/epjconf/201921005011UHECR 2018[9] M. Gottowik, C. Glaser, T. Huege, J. Rautenberg, As-tropart. Phys. 103, 87 (2018)[10] V. Verzi, for the Pierre Auger Collaboration, Proc.33rd ICRC, Rio de Janeiro, Brazil, id 0928 (2013)[11] E. M. Holt for the Pierre Auger Collaboration, Proc.35th ICRC, Busan, Korea, PoS(ICRC2017)492[12] S. Buitink et al. (LOFAR CR KSP), Phys. Rev. D 90,082003 (2014)[13] E. M. Holt for the Pierre Auger Collaboration, Proc.of the 2018 ARENA conference, EPJ WoC in press(2019)[14] T. Huege, A. Haungs, Proc. of the UHECR2014conference, Springdale, USA, JPS Conf. Proc. 09,010018 (2016)[15] A. Aab et al. (Pierre Auger Collaboration), JCAP 10,026 (2018)[16] A. Aab et al. (Pierre Auger Collaboration), ArXiv e-prints, 1604.03637 (2016)[17] J. Hörandel for the Pierre Auger Collaboration, theseproceedings5EPJ Web of Conferences 210, 05011 (2019) https://doi.org/10.1051/epjconf/201921005011UHECR 2018

Radio detection of cosmic rays with the Auger Engineering Radio Array

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Radio detection of cosmic rays with the Auger Engineering Radio Array

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